Complex molecules generally have spectra in the infrared which are rich in detail and specific to the molecule, thereby providing one of the best signatures by which to identify the particular molecule. Various techniques have been developed to measure the infrared spectral absorption or emission characteristic of unknown gases as a means of identifying and quantifying its composition. Advantages and disadvantages of the various methods are described briefly in the specification of U.S. Pat. No. 3,679,899, issued to applicant July 25, 1972. That disclosure, in turn, described a means of reducing the limitations on existing NDIR gas detection devices by providing a means of increasing the specificity based on a technique which could be used with essentially all non-symmetrical molecules of modest size. The method it described, however, required the use of electronic equipment and optical detectors that were more complex than most because the absorption-absorption heterodyning signal on which it was based was inherently small.